1. Field of the Invention
The present invention relates to a measurement device and method for a semiconductor processing system, and more specifically, for a plasma processing system including an apparatus for measuring the uniformity of the plasma at multiple locations adjacent to a surface of a semiconductor wafer being processed.
2. Discussion of the Background
Ion current flux is a parameter that can be used to characterize a plasma in the reaction chamber of a semiconductor wafer processing system. This parameter provides a useful measure of the effectiveness of the plasma in a particular process step and may also, for a suitably designed measurement tool provide information about the uniformity of the plasma at the surface of a wafer being processed.
U.S. Pat. No. 5,801,386 hereinafter “the '386 patent”) describes an ion current probe used to measure ion current flux in a plasma chamber. For many probe configurations, the calculation of the ion current flux density is complicated because the side walls of the current collectors have a thickness typically in excess of one millimeter, which allows the side walls to be exposed to the plasma. Consequently the side walls collect an appreciable ion current, and the calculation of ion current flux density for the placebo wafer must account for the ion current the side walls collect. An additional problem with such probe configurations is that the exposed edges at the juncture of the planar surface and the side walls of the ion current collectors are also a potential source of electrical arcs. Such arcing affects the accuracy of the ion current measurements and may damage the ion current probe as well.
In such current probes, the ion current collectors are usually attached to the placebo wafer with the aid of an adhesive such as an epoxy resin. The attachment is usually not precisely repeatable from placebo wafer to placebo wafer. For example, a collector may not be flat against the surface of the placebo wafer or its location might be slightly displaced from the location of the equivalent ion current collector on another placebo wafer. Consequently, different placebo wafers do not, in general, yield identical data.
In addition, adhesives used to attach the ion current collectors to the placebo wafer are potentially a source of chamber contamination. Furthermore, an adhesive exposed to the plasma may deteriorate physically. In particular, it may become brittle and crack, and the ion current collectors may, consequently, separate from the surface of the placebo wafer.
Moreover, the connecting tail restricts movement of the placebo wafer within the plasma chamber. In addition, it also precludes placing the placebo wafer within or extracting it from the chamber without opening the chamber. The use of a connector tail also makes the measurement less accurate than a measurement made with a truly floating placebo wafer, because the connection to ground influences both the voltage and impedance to ground of the placebo wafer.